nsthread uiviewcontroller interaction

if i parent new thread, following within i pull new controller onto uinavigationcontroller, controlling formula this...

(a) working

-(void)mycallbackinthread
{
    // pierce on...
    uiapplication* app = [uiapplication sharedapplication];
    [app changeview];
}

then i perspective appears, nonetheless does respond user input.

if i change formula this

(b) working

-(void)mycallbackinthread
{
    // pierce on...
    uiapplication* app = [uiapplication sharedapplication];
    [app performselectoronmainthread:@selector(movetomain) withobject:nil waituntildone:false];
}

then all works only fine.

any hints why?